In our preliminary studies, we have established the basic hybridization technique to obtain suppressor T cell hybridomas which produce alloantigen specific and nonspecific suppressor factors. The objectives of the current proposal are 1) to further establish antigenspecific andnonspecific suppressor T cell hybridoma lines using somatic cell hybridization techniques and suppressor T cells obtained from mice unresponsive to skin allografts, 2) to determine the in vitro functions of the hybridoma-derived suppressor factors by mixed lymphocyte cultures and cell-mediated cytotoxicity assays, 3) to characterize the nature of these suppressor factors by immunochemical and physico-chemical analyses, and 4) to assess the capacity of these suppressor factors to prolong skin allograft survival in vivo. We believe that the information obtained from the proposed experiments, when coupled with our previous results, should provide insight into the nature of T cell receptors for alloantigens and the mechanisms responsible for the regulation of cell-cell interactions in allograft unresponsiveness. It is also hoped that such hybridoma lines would serve as a constant source of biologically active suppressor factors which could be used as a biological immunosuppressive reagent for the induction or maintenance of allograft unresponsiveness.